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Rapid Prototyping 

Rapid Prototyping

Rapid Prototyping (RP) can be defined as a group of techniques used to quickly fabricate a scale model of a part or assembly using three-dimensional computer aided design (CAD) data. What is commonly considered to be the first RP technique, Stereolithography, was developed by 3D Systems of Valencia, CA, USA. The company was founded in 1986, and since then, a number of different RP techniques have become available.

Rapid Prototyping has also been referred to as solid free-form manufacturing, computer automated manufacturing, and layered manufacturing. RP has obvious use as a vehicle for visualization. In addition, RP models can be used for testing, such as when an airfoil shape is put into a wind tunnel. RP models can be used to create male models for tooling, such as silicone rubber molds and investment casts. In some cases, the RP part can be the final part, but typically the RP material is not strong or accurate enough. When the RP material is suitable, highly convoluted shapes (including parts nested within parts) can be produced because of the nature of RP.

There is a multitude of experimental RP methodologies either in development or used by small groups of individuals. This section will focus on RP techniques that are currently commercially available, including Stereolithography (SLA), Selective Laser Sintering (SLS®), Laminated Object Manufacturing (LOM™), Fused Deposition Modeling (FDM), Solid Ground Curing (SGC), and Ink Jet printing techniques. 
Why Rapid Prototyping?
The reasons of Rapid Prototyping are

• To increase effective communication.
• To decrease development time.
• To decrease costly mistakes.
• To minimize sustaining engineering changes.
• To extend product lifetime by adding necessary features and eliminating redundant features early in the design.

Rapid Prototyping decreases development time by allowing corrections to a product to be made early in the process. By giving engineering, manufacturing, marketing, and purchasing a look at the product early in the design process, mistakes can be corrected and changes can be made while they are still inexpensive. The trends in manufacturing industries continue to emphasize the following:

• Increasing number of variants of products.
• Increasing product complexity.
• Decreasing product lifetime before obsolescence.
• Decreasing delivery time. 
Rapid Prototyping improves product development by enabling better communication in a concurrent engineering environment. 
Methodology of Rapid Prototyping
The basic methodology for all current rapid prototyping techniques can be summarized as follows:

1. A CAD model is constructed, then converted to STL format. The resolution can be set to minimize stair stepping.
2. The RP machine processes the .STL file by creating sliced layers of the model.
3. The first layer of the physical model is created. The model is then lowered by the thickness of the next layer, and the process is repeated until completion of the model.
4. The model and any supports are removed. The surface of the model is then finished and cleaned.

Highlights of Stereolithography
• The first Rapid Prototyping technique and still the most widely used.
• Inexpensive compared to other techniques.
• Uses a light-sensitive liquid polymer.
• Requires post-curing since laser is not of high enough power to completely cure.
• Long-term curing can lead to warping.
• Parts are quite brittle and have a tacky surface.
• No milling step so accuracy in z can suffer.
• Support structures are typically required.
• Process is simple: There are no milling or masking steps required.
• Uncured material can be toxic. Ventilation is a must. 

Introduction to Stereolithography
Stereolithography (SLA), the first Rapid Prototyping process, was developed by 3D Systems of Valencia, California, USA, founded in 1986. A vat of photosensitive resin contains a vertically-moving platform. The part under construction is supported by the platform that moves downward by a layer thickness (typically about 0.1 mm / 0.004 inches) for each layer. A laser beam traces out the shape of each layer and hardens the photosensitive resin.

The Stereolithography (SLA) System overall arrangement: 

Stereolithography Process
The sequence of steps for producing an Stereolithography (SLA) layer is shown in the following figures:

Uncured resin is removed and the model is post-cured to fully cure the resin. Because of the layered process, the model has a surface composed of stair steps. Sanding can remove the stair steps for a cosmetic finish. Model build orientation is important for stair stepping and build time. In general, orienting the long axis of the model vertically takes longer but has minimal stair steps. Orienting the long axis horizontally shortens build time but magnifies the stair steps. For aesthetic purposes, the model can be primed and painted.

During fabrication, if extremities of the part become too weak, it may be necessary to use supports to prop up the model. The supports can be generated by the program that creates the slices, and the supports are only used for fabrication. The following three figures show why supports are necessary:

Highlights of Selective Laser Sintering
• Patented in 1989.
• Considerably stronger than SLA; sometimes structurally functional parts are possible.
• Laser beam selectively fuses powder materials: nylon, elastomer, and soon metal;
• Advantage over SLA: Variety of materials and ability to approximate common engineering plastic materials.
• No milling step so accuracy in z can suffer.
• Process is simple: There are no milling or masking steps required.
• Living hinges are possible with the thermoplastic-like materials.
• Powdery, porous surface unless sealant is used. Sealant also strengthens part.
• Uncured material is easily removed after a build by brushing or blowing it off. 
Selective Laser Sintering
Selective Laser Sintering (SLS®, registered trademark by DTM™ of Austin, Texas, USA) is a process that was patented in 1989 by Carl Deckard, a University of Texas graduate student. Its chief advantages over Stereolithography (SLA) revolve around material properties. Many varying materials are possible and these materials can approximate the properties of thermoplastics such as polycarbonate, nylon, or glass-filled nylon.

As the figure below shows, an SLS® machine consists of two powder magazines on either side of the work area. The leveling roller moves powder over from one magazine, crossing over the work area to the other magazine. The laser then traces out the layer. The work platform moves down by the thickness of one layer and the roller then moves in the opposite direction. The process repeats until the part is complete.

SLA vs. SLS: A Summarized Comparison
Material Properties: The SLA (stereolithography) process is limited to photosensitive resins which are typically brittle. The SLS® process can utilize polymer powders that, when sintered, approximate thermoplastics quite well.

Surface Finish: The surface of an SLS® part is powdery, like the base material whose particles are fused together without complete melting. The smoother surface of an SLA part typically wins over SLS® when an appearance model is desired. In addition, if the temperature of uncured SLS® powder gets too high, excess fused material can collect on the part surface. This can be difficult to control since there are so many variables in the SLS® process. In general, SLA is a better process where fine, accurate detail is required. However, a varnish-like coating can be applied to SLS® parts to seal and strengthen them.

Dimensional Accuracy: SLA is more accurate immediately after completion of the model, but SLS® is less prone to residual stresses that are caused by long-term curing and environmental stresses. Both SLS® and SLA suffer from inaccuracy in the z-direction (neither has a milling step), but SLS® is less predictable because of the variety of materials and process parameters. The temperature dependence of the SLS® process can sometimes result in excess material fusing to the surface of the model, and the thicker layers and variation of the process can result in more z inaccuracy. SLA parts suffer from the "trapped volume" problem in which cups in the structure that hold fluid cause inaccuracies. SLS® parts do not have this problem.

Support Structures: SLA parts typically need support structures during the build. SLS® parts, because of the supporting powder, sometimes do not need any support, but this depends upon part configuration. Marks left after removal of support structures for parts cause dimensional inaccuracies and cosmetic blemishes.

Machining Properties: In general, SLA materials are brittle and difficult to machine. SLS® thermoplastic-like materials are easily machined.

Size: SLS® and SLA parts can be made the same size, but if sectioning of a part is required, SLS® parts are easier to bond.

Investment Casting: The investment casting industry has been conservative about moving to RP male models, so SLS® models made from traditional waxes, etc. are preferred. 3D Systems has a process (dubbed "QuickCast™") which allows SLA models to be more suitable for investment casting. Since SLA resins do not melt but burn to form ash, QuickCast™ modifies the build process so that the interior of the model is hollow with a supporting latticework. When the ceramic is fired, the QuickCast™ model collapses and any ash is minimal because of the small total quantity of material.

Highlights of Fused Deposition Modeling
• Standard engineering thermoplastics, such as ABS, can be used to produce structurally functional models.
• Two build materials can be used, and latticework interiors are an option.
• Parts up to 600 × 600 × 500 mm (24 × 24 × 20 inches) can be produced.
• Filament of heated thermoplastic polymer is squeezed out like toothpaste from a tube.
• Thermoplastic is cooled rapidly since the platform is maintained at a lower temperature.
• Milling step not included and layer deposition is sometimes non-uniform so "plane" can become skewed.
• Not as prevalent as SLA and SLS®, but gaining ground because of the desirable material properties. 
Fused Deposition Modeling
Stratasys of Eden Prairie, MN makes Fused Deposition Modeling (FDM) machines. The FDM process was developed by Scott Crump in 1988. The fundamental process involves heating a filament of thermoplastic polymer and squeezing it out like toothpaste from a tube to form the RP layers. The machines range from fast concept modelers to slower, high-precision machines. The materials include polyester, ABS, elastomers, and investment casting wax. The overall arrangement is illustrated below:

Highlights of Solid Ground Curing
• Large parts, 500 × 500 × 350 mm (20 × 20 × 14 in), can be fabricated quickly.
• High speed allows production-like fabrication of many parts or large parts.
• Masks are created w/ laser printing-like process, then full layer exposed at once.
• No post-cure required.
• Milling step ensures flatness for subsequent layer
• Wax supports model: no extra supports needed.
• Creates a lot of waste.
• Not as prevalent as SLA and SLS, but gaining ground because of the high throughput and large parts.

Solid Ground Curing: An Introduction
Solid Ground Curing, also known as the Solider Process, is a process that was invented and developed by Cubital Inc. of Israel. The overall process is illustrated in the figure above and the steps are illustrated below. The SGC process uses photosensitive resin hardened in layers as with the Stereolithography (SLA) process. However, in contrast to SLA, the SGC process is considered a high-throughput production process. The high throughput is achieved by hardening each layer of photosensitive resin at once. Many parts can be created at once because of the large work space and the fact that a milling step maintains vertical accuracy. The multi-part capability also allows quite large single parts (e.g. 500 × 500 × 350 mm / 20 × 20 × 14 in) to be fabricated. Wax replaces liquid resin in non-part areas with each layer so that model support is ensured.

Solid Ground Curing Process

The steps in the process are as follows.

First, a CAD model of the part is created and it is sliced into layers using Cubital's Data Front End® (DFE®) software. At the beginning of a layer creation step, the flat work surface is sprayed with photosensitive resin, as shown below:

For each layer, a photomask is produced using Cubital's proprietary ionographic printing technique, as illustrated below:

Next, the photomask is positioned over the work surface and a powerful UV lamp hardens the exposed photosensitive resin:

After the layer is cured, all uncured resin is vacuumed for recycling, leaving the hardened areas intact. The cured layer is passed beneath a strong linear UV lamp to fully cure it and to solidify any remnant particles, as illustrated below:

In the fifth step, wax replaces the cavities left by vacuuming the liquid resin. The wax is hardened by cooling to provide continuous, solid support for the model as it is fabricated. Extra supports are not needed.

In the final step before the next layer, the wax/resin surface is milled flat to an accurate, reliable finish for the next layer:

Once all layers are completed, the wax is removed, and any finishing operations such as sanding, etc. can be performed. No post-cure is necessary.

Ink Jet Printing Techniques

Ink jet printing comes from the printer and plotter industry where the technique involves shooting tiny droplets of ink on paper to produce graphic images. RP ink jet techniques utilize ink jet technology to shoot droplets of liquid-to-solid compound and form a layer of an RP model. Common ink jet printing techniques, such as Sanders ModelMaker™, Multi-Jet Modeling™, Z402 Ink Jet System™, and Three-Dimensional Printing, are presented in this section. Although none of the these techniques have become as established as the Stereolithography (SLA) or Selective Laser Sintering (SLS®) systems, several show promise. 
Sanders ModelMaker

• Exceptional accuracy allows use in the jewelry industry.
• Accuracy is partly enabled by a milling step after each layer deposition.
• Plotting system is a liquid-to-solid inkjet which dispenses both thermoplastic and wax materials.
• Compared to SLS® and SLA, not as established. 
The Sander ModelMaker™ product is produced and distributed by Sanders Prototype, Inc. of Wilton, NH, USA. Smooth cosmetic surface quality can be achieved by pre-tracing the perimeter of a layer prior to filling in the interior. The supporting wax material is deposited at the same time as the thermoplastic. A schematic is shown below:

Both the thermoplastic material (Protobuild™) and the wax support material (Protosupport™) are proprietary materials of Sanders. 
Multi-Jet Modeling

• Fast.
• Office-friendly: non-toxic materials, small footprint, low odor.
• Simple operation: operates as a network printer in an office environment.
• Models are primarily for appearance use.
• Compared to SLS® and SLA, not as established. 
Another product of 3D Systems from the makers of the SLA system, Multi-Jet Modeling™ uses a 96-element print head to deposit molten plastic for layering. The system is fast compared to most other RP techniques, and produces good appearance models with minimal operator effort. The main market that this system is targeted at is the engineering office where the system must be non-toxic, quiet, small, and with minimal odor. The system is illustrated below: 

Z402 Ink Jet System

• Fast: one to two vertical inches per hour, depending on layer density.
• Office-friendly: non-toxic materials, small footprint, low odor.
• Simple operation.
• Compared to SLA and SLS®, not as established. 
The Z402™ is one of the fastest 3D printers known to Rapid Prototyping. The ability to produce quick models means greater productivity for the lab and quick prototypes for customers. Since manufacturing parts is easy, almost anyone in the lab can produce a quality part without extensive Rapid Prototyping experience. 

Three-Dimensional Printing
• Binder is "printed" on unbound powder layer.
• Without milling step, work plane can become successively skewed.
• Not as established as SLA and SLS®. 
Three-Dimensional Printing, developed by MIT and Soligen, Inc., is illustrated below. It is another technique based on the inkjet printing process. Binder is printed on a powder layer to selectively bind powder together for each layer.

Rapid Tooling: An Introduction

The term Rapid Tooling (RT) is typically used to describe a process which either uses a Rapid Prototyping (RP) model as a pattern to create a mold quickly or uses the Rapid Prototyping process directly to fabricate a tool for a limited volume of prototypes. RT is distinguished from conventional tooling in that:
a) Tooling time is much shorter than for a conventional tool. Typically, time to first articles is below one-fifth that of conventional tooling.

b) Tooling cost is much less than for a conventional tool. Cost can be below five percent of conventional tooling cost.

c) Tool life is considerably less than for a conventional tool.

d) Tolerances are wider than for a conventional tool.
We do not intend to comprehensively cover all types of RT in this narrative. In addition to Silicone Rubber Molding (SRM), we present Composite Molding and Direct AIM (ACES Injection Molding). The field of RT is expanding rapidly and information on many of the new methodologies is still changing.

Source : www.efunda.com

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